The invention relates to an oil supply system for a gas turbine having a number of combustion chambers being able alternatively to be fired with gas or with oil, and equipped with primary and secondary nozzles. The system comprises an oil feeding device, a water feeding device and a blow-off device. The oil feeding device is provided with an oil supply line with a fuel distributor and a number of primary oil-conducting conduits leading from the fuel distributor to the combustion chamber to deliver oil to the primary nozzles and of secondary oil-conducting conduits running parallel to the primary oil-conducting conduits leading to the secondary nozzles. The water feeding device is provided with a water supply line and a number of primary water-conducting conduits and secondary water-conducting conduits corresponding to the number of combustion chambers and connected to the water supply line via a distributor, to deliver water to the primary and secondary nozzles. The blow-off device is provided with an air supply line split into two supply branch lines and with two closed circular pipes connected to the supply branch linen via connection valves. A number of primary air-conducting conduits and secondary air-conducting conduits corresponding to the number of gas turbine combustion chambers deliver blow-off air to the primary and secondary nozzles.
To be able to comply with the often very restrictive emission standards in case of gas turbines that can alternatively be fired with gas or oil, these gas turbines are operated according to the Dual-Fuel-Dry-Low-Nox (DLN) process, in which thermal Nox is already reduced in the combustion chamber by a better mixing of gas and air. This presupposes a multi-nozzle system of primary nozzles and secondary nozzles for each combustion chamber. In case of oil firing, the NOx reduction is achieved by the injection of water for flame cooling, with water and oil being mixed immediately upstream of the gas turbine combustion chambers or injected separately into the combustion chambers. If turbine operation is changed from oil to gas firing, each oil and water duct of each combustion chamber nozzle, air must be blown through each nozzle to remove oil residues and to provide for a continuous duct cooling. Therefore, the gas turbine requires an oil supply system for oil firing that comprises an oil feeding device, a water feeding device and a blow-off device and that, compared with gas firing, presents considerable difficulties as far as control and the distribution of fuel are concerned.
For oil supply, the burning oil delivered via an oil pump has heretofore been pumped through a fuel distributor to individual primary oil-conducting conduits leading to the gas turbine combustion chambers for the oil delivery to the primary nozzles. The primary oil-conducting conduits have been branched off the secondary oil-conducting conduits for the delivery of oil to the secondary nozzles. Secondary valves have permitted connection and disconnection of the secondary oil-conducting conduits, as the secondary nozzles are fired only after a certain firing temperature is reached. Till then, the whole fuel quantity is delivered to the primary nozzles. Afterwards, however, the fuel quantity must be distributed to the primary and secondary nozzles at a certain ratio. Due to branching the secondary oil conduits off the primary oil conduits, however, fuel distribution depends on the resistances in the supply lines and fuel dosing is uncontrolled, which time and again leads to thermal overloads and/or irregular loads of the individual gas turbine combustion chambers and/or the combustion chamber nozzles, the flame tubes, cross ignition tubes and the like, and may even cause a total wreckage of the turbine.
To reduce the NOx emissions the in oil firing operation, water must be injected at a certain quantitative proportion to oil for flame cooling, whereby thermal NOx can be reduced. To this end, deionized water is delivered to the primary and secondary nozzles by means of an appropriate jetting pump via multiple connection valves. The water injection starts only after firing temperature is reached, when the secondary nozzles are connected to the primary nozzles. Water injection is controlled via a flap valve that first reduces the pump pressure to the desired injection pressure, whereupon a control valve controlled by a water meter determines the quantity of water as a function of the oil quantity, to comply with the maximum admissible quantity ratio water/oil of, for instance, 0.85. In this case, too, there are high load variations due to the flap valve, mainly during the beginning of water injection. Because of the use of single connection valves, due to the different line resistances, there is an irregular distribution of the overall water quantity to the individual gas turbine combustion chambers with irregular loads of the combustion chambers and/or their parts.
When changing the operating mode from oil to gas firing, the oil and water ducts of the combustion nozzles must be cooled with air. Additionally, the residual oil remaining in the combustion nozzles must be blown off for cleaning and to prevent coking. Therefore, there is a blow-off device with corresponding primary and secondary air-conducting conduits. The blow-off air is tapped from the gas turbine compressor and then, by means of a separate mechanically driven air compressor, its pressure is increased, for instance, to 1.4 times the compressor pressure for blowing against the combustion chamber pressure. When changing to gas firing takes place at a firing temperature, where primary and secondary nozzles are operated, both the primary and the secondary air conducting conduits are all of a sudden connected via mere on/off valves, whereby the residual oil quantities are blown into the combustion chamber all of a sudden too. This creates a sudden increase of power, which, in turn, entails load variations affecting the service life of the turbine, endangers availability due to a turbine failure and even may lead to a shut-down of the turbine due to the over temperature in the combustion chambers.
The invention has therefore the object to provide an oil supply system of the above mentioned kind that ensures a sound oil firing operation of the gas turbine, complying with the emission standards even when the operating mode is changed, and furthermore ensures that firing of the turbine is easy on the combustion chambers and their parts.
The object of the invention is achieved according to one aspect thereof in that the oil feeding device comprises a secondary fuel distributor for the secondary oil-conducting conduits in addition to the primary fuel distributor for the primary oil-conducting conduits. The secondary fuel distributor is also connected to the oil supply line and the fuel distributors deliver quantities of oil in an adjustable distribution ratio through the oil-conducting conduits to the combustion chambers. According to another aspect, water distributors, such as variable-speed distribution pumps, with an adjustable distribution ratio are provided in the water feeding device as distributors for the primary and secondary water-conducting conduits. According to still another aspect, the blow-off device is equipped with proportional minimum pressure valves for the supply branch lines.
Due to the primary fuel distributor and the secondary fuel distributor distributing the delivered oil quantity exactly to the primary oil-conducting conduit and the secondary oil-conducting conduit according to a certain adjustable distribution ratio such as 60:40, an exact dosing of the oil quantity is ensured both for the primary nozzles and for the secondary nozzles of the combustion chambers. The fuel distributor is a positive fuel distributor such as a gear-type fuel distributor or a piston-type fuel distributor. Therefore, the fuel quantity combusted by the primary and/or secondary nozzles is exactly predetermined for each power range, thus avoiding thermal overloads of combustion nozzles, flame tubes and the like. As combustion is evened out, the service life as well as the availability of the gas turbine is increased and, last but not least, the regulation of water feeding for the necessary flame cooling and NOx reduction is facilitated.
By the application of water distributors which, again, are positive distributors, such as variable-speed distribution pumps for the primary water-conducting conduits and secondary water-conducting conduits, the water feeding device also permits an exact quantity distribution at the given distribution ratio to the primary nozzle and secondary nozzle. Additionally, the water quantity is dosed as a function of the oil quantity delivered to the combustion chambers. Moreover, a feeder pump of a lower pressure level upstream of the water distributors, such as distribution pumps, is sufficient, whereby the dosed water quantities can be increased and/or reduced gently and regularly from 0 to a maximum quantity. Thus, the gas turbine operation is evened out further, and during the whole period of oil firing the required reduction of thermal NOx is ensured, which produces a further preservation of the turbine parts and a corresponding extension of service life.
By the installation of one proportional minimum pressure valve each in the supply branch lines for the primary and secondary air-conducting conduits, the blowing-off pressure, which must be considerably higher than the combustion chamber pressure, can be increased slowly from an adjustable preliminary pressure that is equal to the combustion chamber pressure up to the final blowing-off pressure, for instance 1.4 times the combustion chamber pressure, with the time of changes being adapted to the actual conditions. Therefore, when changing the operating mode from oil firing to gas firing, the air pressure is increased all of a sudden to the adjustable preliminary pressure by means of the proportional minimum pressure valves, and then is increased steadily to the final blowing-off pressure. Thus, the residual fuel quantity is blown only slowly from the combustion nozzles into the combustion chamber, whereby an undesired increase of power is avoided. Thus, the conversion of the operating modes proceeds gently, which, in turn, extends the service life of the turbine and considerably increases its availability.
It is particularly advantageous if a switching valve is arranged in each secondary oil-conducting conduit, with a connection to the supply line to a primary oil-conducting conduit, on the one hand, and to a return line to an oil pan, on the other hand. This switching valve may be a multiple switching valve unit for all secondary oil-conducting conduits and permits the connection of the secondary oil-conducting conduits to the primary oil-conducting conduits at a firing temperature below a certain minimum temperature, so that the whole oil quantity is delivered to the primary nozzles. As soon as the minimum temperature is reached, the multiple switching valve unit opens the secondary oil-conducting conduits while simultaneously disconnecting the supply lines to the primary oil-conducting conduits, whereby the primary and secondary nozzles of all combustion chambers receive exactly the required fuel quantities at the preset distribution ratio of the primary and secondary fuel distributors. If the switching valves switch the return lines to the oil pan, the supply lines to the primary oil-conducting conduits may open into the oil pan. Thus it is prevented that, when changing the operating mode, the oil locked up in the primary and secondary conduits is pressed unchecked into the combustion chamber, whereby the CO emission of the gas flame would be extremely increased, thus endangering compliance with the emission standards.
To prevent dripping of oil and water and to interrupt it all of a sudden upon a change of the operating mode form oil to gas firing, check valves are arranged in each oil, water and air conduit to the combustion nozzles. The check valves also prevent the penetration of oil and water into the blow-off air-conducting conduits. Flat-type check valves have been used as check valves that have been screwed into each supply line immediately upstream of the combustion nozzles. To keep these valves running well, the gas turbine has had to be regularly converted to oil firing according to the operating instructions, which has brought about an accumulation of the drawbacks of a conversion of the operating mode. Moreover, the number of screwed connections have not only been complicated and time-consuming in handling but due to leaks, there has also been the danger of fire. The flat-type check valves easily coked due to their continuous contact with the water and oil and due to the high temperatures, which time and again has led to blockages of the check valves or to inadequate closing, This brought about a varying mass flow rate with the risk of burning down of the combustion nozzle flame tubes, and the unchecked input of residual oil or water into the combustion chamber endangering the emission standards. Things have been similar with primary distributor valves distributing the oil supply to the primary oil-conducting conduits leading to the primary nozzles of each combustion chamber. To avoid these drawbacks, a primary valve block with connection bores for the primary oil, water and air conduits to be connected via the check valves is arranged upstream of the primary nozzles of a combustion chamber, with air scavenging ducts connecting the air connection bores with the oil and water connection bores. Analogously, a secondary valve block with connection bores for the secondary oil, water and air conduits to be connected via the check valves is arranged upstream of the secondary nozzle of each combustion chamber, with air scavenging ducts connecting the air connection bores with the oil and water connection bores. These valve blocks make is possible that the return valves are joined to one manifold block and to use ball check valves instead of flat-type check valves. Moreover, the blow-off air is conducted from the air connection bore to the oil and water connection bores via appropriate air scavenging ducts, where it scavenges the ball check valves with blow-off air, thus preventing their coking or similar malfunction. By using scavenged and cooled ball check valves, blocking or inadequate closing of the check valves is prevented, so that the requirement for regular conversions of the operating mode to ensure proper functioning of the valves is no longer necessary. This leads to enormous cost reductions and to an increase of availability. Moreover, the valve blocks facilitate handling and installation of the valves, the danger of leaks is reduced to a minimum and the available space in the area of combustion chamber piping is extended accordingly.
As on/off valves are frequently used in the oil-conducting conduits that are triggered hydraulically via control valves, there has been the risk that, when changing the operating mode from oil firing to gas firing, the control valves, usually slide valves, that are no longer actuated get conglutinated due to the thermal load and are no longer fit for use after a reconversion of the gas turbine to oil firing. If control valves are provided for a hydraulic control of on/off valves in the oil-conducting conduit, the control valves may be periodically actuated with the control lines between the control and the on/off valves closed, to ensure controllability in spite of the risk of conglutination, so that, via a repeated actuation of the control valves, their fitness for use can be ensured. Off course, during actuation of the control valves, the control lines leading to the on/off valves must be closed to exclude switching errors.
Another improvement of the blow-off effect can be achieved by joining the oil and water connection bores to one feeding bore branching into several sub bores ending in a distribution chamber at its circumference, from where nozzle ducts lead to the primary nozzles via a distribution valve. A primary zone casing is preferably inserted in the distribution chamber. Splitting up of the feeding bores into sub bores prevents the accumulation of major residual oil quantities in lines of large cross sections. The air blown into the distribution chamber at its circumference will thus cover the whole distribution chamber and blow it clear. This is further improved by the primary zone casing on the distribution chamber, as dead space is eliminated due to the wedge-shaped cross section of the primary zone casing. This reduces residual oil quantities to a minimum. Additionally, since the valve block keeps the flow distances to the combustion nozzles as short as possible, the residual oil quantity is further reduced and blowing-off of this residual oil quantity is facilitated.